Liquid propellants may be desirable as propellant, or fuel and oxidizer sources for rocket engines, as liquids have a relatively high density that may allow the volume of the tanks to be relatively small, which may result in a relatively high mass ratio. Additionally, rocket engines utilizing liquid propellants may be configured to be fueled immediately prior to launch, launched, recovered and then later refueled and reused.
Liquid fueled rocket engines may be generally categorized as open-cycle engines or closed-cycle engines. An open-cycle engine may utilize a gas-generator. The gas generator burns propellant to generate a hot gas to drive a turbine that is used to power the engine's pumps. The gases from the gas generator are then exhausted, separately from a nozzle of the engine, thus the use of the term “open-cycle.” The separate discharge of exhaust gases generated by the pre-burner results in impulse losses to the rocket.
In closed-cycle engines, such as staged combustion cycle engines, all of the propellant gases may be expelled through a nozzle of the engine, which may result in a more efficient engine. However, staged combustion cycle engines still require a pre-burner to generate hot gases to drive a turbine to power the engine's pumps. This may result in a relatively complex engine design, relatively harsh operating conditions for the turbine and may require specialized plumbing to carry the hot gases through the system. Additionally, a staged combustion power cycle may result in unnecessary pressure build-up, which results in system inefficiencies. For example, a staged combustion rocket engine assembly may have a pump discharge to combustion chamber pressure ratio between 2.5 and 3.
Expander cycle engines heat liquid fuel to form a vaporized fuel that is utilized to drive a turbine to power the engine's pumps. In view of this approach, expander cycle engines do not require a gas generator or pre-burner. However, conventional closed-cycle expander engines, which direct all of the fuel through the turbine and then into a combustion chamber of the engine, require a pump discharge to combustion chamber pressure ratio between 2.5 and 3. This additional pressure build-up requires relatively high turbine power and turbine inlet pressure. These factors lead to weight increase and lower reliability. Open-cycle expander engines, which only heat some of the fuel to drive a turbine and then discharge the heated fuel to improve turbine efficiency, may not require the mentioned pressure build-up and related implications compared to a conventional closed-cycle expander cycle engine. However, the discharging of a portion of the fuel results in efficiency losses.
In view of the foregoing, improved rocket engine assemblies, rocket engine power cycles and methods of operating rocket engines would be desirable.